2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is characterized as one of the most toxic chemicals known to man. Its toxic effects include porphyria, a severe wasting syndrome, metaplasia, teratogenesis and immune suppression. The aryl hydrocarbon receptor (AHR) is essential to this toxicity. In the absence of ligand, the AHR is found in the cytosol bound to a dimmer of Hsp90 and a monomer of ARAS. ARA9 shares sequence similarities to FK506 binding proteins. ARA9 acts to maintain stability and proper cellular compartmentalization of the AHR. The method by which it fulfills these functions is unknown. In addition, what role ARA9 plays in secondary signaling and toxicity of ligands for the AHR has not been determined. Finally, it is not known whether ARA9 can recruit other cellular proteins to the AHR cytosolic complex or act to influence other cellular signaling cascades. Preliminary data suggests that ARA9 may interact with a known mediator of various signaling pathways. This interaction has lead to the following hypothesis: ARA9 influences dioxin mediated signaling by recruiting other proteins to the AHR cytosolic complex and directly influencing other cellular signaling cascades. To address this hypothesis, three specific aims are proposed: 1) Map the interaction surfaces of ARA9 and other cellular signaling molecules. 2) Use proteomic tools to identify new members of the AHR signaling network. 3) Identify changes in phosphorylation patterns in TCDD treated cells using a novel phosphoproteomic solid phase enrichment technology. Upon completion of these aims, the investigators will have extended the understanding of TCDD toxicity by identifying other cellular targets for signaling. In addition, they will have directly related a significant post-translational modification to this signaling cascade. Finally, this work will establish a potential mechanism for secondary toxicity as it relates to TCDD exposure.